Methods of electrolytically co-precipitating mixed metal oxides are known from Canadian Patent No. 593 187 and U.K. Patent No. 864 249. Another method of precipitating or co-precipitating metal compounds, more particularly oxides and hydroxides, in an electrolysis cell is described in Canadian Patent No. 623 339.
It was recognized in the latter patent that the process should be carried out at a specific and as constant as possible pH, and this was achieved by carrying out a preliminary electrolysis to obtain suitable starting conditions and setting the electrolysis current at a value corresponding to the rate of supply of a solution of the salts from which the compounds are precipitated. In carrying out these known processes, it has been observed that the particle size and density do not remain constant and this reduces the usefulness of the product. The invention is based on the observation that the uneven particle size and variable density obtained with the known processes are due to variations in the solution pH which occur when following the prior teachings and because of the inherent difficulties in measuring and controlling the pH in an electrolysis cell particularly when gas is bubbled in or is electrochemically generated. Similar considerations apply to the control of the particle size and density of electrodeposited coatings and to the control of electrophoretic deposition.